The thermally induced changes of the magnetic coupling in an artificial antiferromagnet exchange biased by an antiferromagnetic layer in a magnetic tunnel junction (Mn-Ir/Co-Fe/Ru/Co-Fe/AlOx/Ni-Fe) is investigated for annealing temperatures up to 450degreesC. Beside the usual increase of the tunneling magnetoresistance (TMR) by annealing (maximum TMR 43.9% at 325degreesC) the degradation of the artificial antiferromagnet causes a well-defined 90degrees coupling of the Co-Fe layers in a narrow temperature window around 325degreesC. At higher temperature, both Co-Fe layers are coupled ferromagnetically but the exchange biasing by the Mn-Ir is still present. This behavior results from an interplay of different coupling mechanisms of the two Co-Fe layers: (1) indirect antiferromagnetic interlayer exchange coupling across the Ru spacer, (2) ferromagnetic coupling by pinholes in the Ru spacer, and (3) exchange coupling between the lower Co-Fe layer and the antiferromagnet Mn-Ir. The importance of each contribution is discussed with respect to thermally activated diffusion processes. (C) 2003 American Institute of Physics.